There are also four fundamental forces in the universe: the gravitational force, the electromagnetic force, the weak nuclear force, and the strong nuclear force. At the microscopic level, each force has an associated particle: the graviton, the photon, the boson and the gluon, respectively. You can think of each of these particles as the smallest packets of any given force, somewhat like the atom was once considered to be the smallest packet of matter (though we now know that this is not true).

We are most familiar with the gravitational force. It is the force that keeps the planets orbiting the sun, the sun orbiting the center of the galaxy, and the force that holds you and me to the Earth. Gravity is always attractive, and the force of gravity is directly proportional to the mass of the two interacting objects, while inversely proportional to the square of the distance between the two objects. Gravitons are the only particles mentioned above which have not been experimentally verified, however physicists are fairly certain that they do exist.

We are also familiar with the electromagnetic force. You have experienced this force when you get a small shock on a dry day from touching metal, when you power up your computer, or when you witness a lightning storm. The electric charge of an object, when dealing with the electromagnetic force, is analogous to the mass of an object when dealing with the gravitational force. The electromagnetic force can be attractive or repulsive. An object with a net positive charge will repel another object with a net positive charge, and attract an object with a net negative charge.

Strong and weak nuclear forces are less familiar to us because they operate on a subatomic level. Though they are extraordinarily strong forces, their strength rapidly diminishes as the distance between two objects increases. The strong force is responsible for holding the quarks inside a neutron and a proton together, and holding the nucleus of an atom together while the weak force is best known for causing radioactive decay. Particles have intrinsic strong and weak charges that determine how the strong and weak force between two particles will interact.

There are huge discrepancies in the relative strengths of the forces. For instance, the electromagnetic force is tredecillion (10^42) times stronger than the gravitational force. Why? If the relative strengths between the forces were even slightly different, the universe would be a much different place. Why is gravity only attractive? Why does the influence of gravity and the electromagnetic force extend over large distances whereas the influence of the strong and weak nuclear force quickly diminishes over distances. String theory again offers a plausible explanation for these questions. Not only that, it unifies the four forces into one, something that Einstein spent the latter years of his life diligently working on.